1. Field of the Invention
The present invention relates to a technology for protecting the surface of an image carrier used to form an electrophotographic image by using a protecting agent for the image carrier.
2. Description of the Related Art
Conventionally, image formation based on an electrophotographic system is implemented by forming an electrostatic latent image on an image carrier that has a photoconductive layer containing a photoconductive material or the like, and attracting charged toner to the electrostatic latent image to form a visible image. The visible image is transferred to a recording medium such as a sheet of paper and is fixed on the recording medium by heat, pressure, or solvent gas, and an output image is thereby obtained.
The image formation is roughly classified into a two-component developing method of using frictional charging due to stirring and mixing of toner and carrier, and a one-component developing method of charging toner without using carrier, depending on how to charge toner to obtain a visible image. The one-component developing method is further classified into a magnetic one-component developing method and a nonmagnetic one-component developing method depending on whether a magnetic force is used to hold the toner on a developing roller.
The two-component developing method is commonly used for copiers that require high speed capability and image reproduction or for multifunction products based on the copier because of requirements such as charging stability of toner, rising capability, long-term stability of the image quality. On the other hand, the one-component developing method is commonly used for compact type printers and facsimiles that require space saving and low cost.
Recently, colorization of output images is widely spread, and therefore, in both of the developing methods, requirements for higher quality of images and stability of image quality are increasing more and more. To achieve the higher quality of images, an average particle size of toner is made smaller and an angular portion of its particle shape is made smoother, so that toner is becoming more spherical.
In general, irrespective of different developing methods, an electrophotographic image forming apparatus uniformly charges a drum-shaped or a belt-shaped image carrier while being made to rotate, forms a latent image pattern on the image carrier by using laser light or the like, visualizes the latent image pattern with toner (toner image), and transfers the toner image to the recording medium. After the toner image is transferred to the recording medium, toner components not having been transferred remain on the image carrier. If these residues are conveyed to a charging process without being removed, these residues prevent uniform charging on the image carrier. Therefore, after the transfer process, the toner or the like remaining on the image carrier is generally removed by a cleaning unit such as a cleaning blade, to make the surface of the image carrier be sufficiently clean, and thereafter the image carrier is charged.
The surface of the image carrier is physically stressed and electrically stressed in various manners during each process of charging, development, transfer, and cleaning, and the state of the surface changes in association with used hours. The stress due to friction in the cleaning process of these stresses causes the image carrier to wear and to be scratched. To resolve these problems, many solutions have been proposed such as various types of lubricants, supply of a lubricating component, and a method of forming a lubricant layer.
To extend life of a photoconductor and a cleaning blade, Japanese Patent Application Laid-Open No. S51-22380 (Patent document 1) proposes a technology for supplying a solid lubricant that contains zinc stearate as a main component to the surface of a photoconductor to form lubricant coating thereon.
Japanese Patent Application Laid-Open No. 2005-274737 (Patent document 2) describes that by using a lubricant applying device that applies a lubricant containing higher alcohol having a carbon number of 20 to 70, the higher alcohol stays as amorphous particles at an edge of a blade nip and this causes the surface of an image carrier to become appropriately wet, and thus lubricating capability is continued.
Japanese Patent Application Laid-Open No. 2002-97483 (Patent document 3) describes that by using particular powder of alkylene bis-alkyl acid amide compound as a lubricating component, there exists the powder on an interface between a cleaning blade and the surface of an image carrier, which allows smooth lubricating effect to be maintained over a long period of time.
However, as explained above, the stress on the image carrier is derived not only from the cleaning process but also includes electrical stress particularly in the charging process that largely changes the state of the surface of the image carrier. The electrical stress is accompanied with an electrical discharge phenomenon near the surface of the image carrier, and this phenomenon is significant in a contact charging system and a proximity charging system. In these charging systems, many active species and reaction products are produced on the surface of the image carrier, and the active species and the reaction products produced in the air of a discharging area are commonly attracted to the surface of the image carrier.
Therefore, the lubricant using the zinc stearate as described in Patent document 1 comparatively evenly covers the surface of the image carrier to give an appropriate lubricating property thereto. However, if a lubricant layer produced thereby repeatedly passes through the charging process, the stearic acid is decomposed and may eventually remain as zinc oxide on the surface of the image carrier and the surface of a charging unit. The remaining zinc oxide has moisture absorption characteristics, and the resistance reduces caused by moisture absorption in the air. Therefore, electrostatic charge on the image carrier cannot be retained under high humidity environment, and an electrostatic latent image becomes vague, which may cause image failure so-called image blur to occur.
The lubricant based on the higher alcohol described in Patent document 2 is easy to wet the surface of the image carrier and the effect as the lubricant can be expected. However, an area occupied by each of higher alcohol molecules absorbed in the image carrier tends to increase, and the density of molecules absorbed in the image carrier per unit area (weight of absorbed molecules per unit area) is low. Consequently, the electrical stress easily penetrates a protecting layer, and thus the effect to satisfactorily protect the image carrier cannot easily be obtained.
As described in Patent document 3, the lubricant containing nitrogen atoms in molecules produces an ionic dissociating compound as a decomposed product like a nitrogen oxide and an ammonium-containing compound when the lubricant itself is exposed to the electrical stress. And the ionic dissociating compound is taken into the lubricant layer and the resistance of the lubricant layer is reduced under high humidity, which may cause image blur to occur.
Long-life image forming apparatuses and long-life components for use in the image forming apparatuses have the great interest at the market in terms of reduction of running costs and protection of global environment due to reduction of waste. For example, to achieve a long-life image carrier, Japanese Patent Application Laid-Open No. 2004-302451 (Patent document 4) discloses a trial in such a manner that a specific surface layer having a cross-linked structure is provided on the surface of the image carrier to improve mechanical durability.
However, as explained above, if a low-resistance substance is taken into the lubricant layer of the image carrier and if the low-resistance substance is removed thereafter, it will be necessary to scrape the whole lubricant layer by, for example, a cleaning mechanism. However, because the lubricant layer itself is slippery, not only the large force is needed for its removal but also the large mechanical stress is applied to the image carrier upon the removal. Therefore, even if the specific surface layer having the cross-linked structure is provided on the surface of the image carrier as described in Patent document 4, the provision of the surface layer does not lead to the long-life image carrier.
Recently, polymerized toner particles manufactured by using a polymerization method are regarded as important to improve image quality and reduce manufacturing energy. The polymerized toner has excellent features such that the polymerized toner particles have angular portions less than these of pulverized toner particles manufactured by a pulverization method, and have a small average particle size and the particles are uniform. However, in the method of pressing an edge of a cleaning unit such as a rubber-made cleaning blade against the surface of the image carrier to clean the surface thereof, it becomes difficult to block the toner particles by the edge due to the shape and the particle size of the polymerized toner particles, which easily causes cleaning failure to occur, namely, residual toner components cannot satisfactorily be cleaned.
Japanese Patent Application Laid-Open No. 2000-330441 (Patent document 5) proposes an image forming apparatus in which a cleaning device capable of improving cleaning failure of such toner as explained above sets a pressing force to satisfy predetermined conditions using a volume-average particle size D and average circularity S of the toner. It is described in this proposal that if a pressing force f is increased when a counter-type cleaning blade is used, some trouble such as squeaking of the cleaning blade and a warp thereof occurs, and it is therefore necessary to set an upper limit as an experimental value.
Furthermore, Japanese Patent Application Laid-Open No. 2005-99125 (Patent document 6) proposes a cleaning device in which a frictional coefficient between toner and an image carrier, a frictional coefficient between toner and a blade, an adhesion force between the toner and the image carrier, a force by the blade against the toner, and an angle (cleaning angle) formed between the blade and the image carrier are respectively defined to clean the toner having a smaller average particle size and a shape closer to a sphericity.
Patent documents 5 and 6 propose the devices to improve the cleaning capability of spherical toner represented by the polymerized toner while reducing the stress on the image carrier in the cleaning mechanism, but there is neither disclosure nor suggestive hint about longer operating life in consideration of the electrical stress on the image carrier. Thus, it does not seem to improve the capability.
Therefore, even though the protection of the surface of the image carrier from the electrical stress in the charging process is extremely important to the longer operating life of the image carrier and the charging unit and to the stabilized image quality, appropriate studies on this matter have not been conducted until now, and at the present situation, this matter therefore still remains as an unsolved problem.